Tidal stream development and its effect on the ... - Springer Link

Hydrobiologia, vol. 42, 4, pag. 369-379,

1973.

Tidal Stream Development and Its Effect on the Distribution of the American Oyster

The distribution of the American oyster is dependent upon the morphological development of tidal streams. Dense populations are most often found associated with large meanders. The differential current velocities associated with meander formation result in areas of scour, which are beneficial to development of oyster beds; and areas of deposition, which are detrimental to survival of oysters.

This research was undertaken to determine the effect of tidal stream development on the distribution of the American oyster Crassostrea uirginica (GMELIN). GRAVE(1905) offered one of the earliest hypotheses on the origin and manner of formation of reefs by the American oyster. We agree with his concept of oysters gradually building out from the shore towards the main channel of a tidal river or stream. However, this process is closely associated with development of the tidal stream per se and for this reason an expansion of GRAVE'S initial concept is warranted. I n 1957 the oyster parasite MSX, Minchinia nelsoni (HASKIN, STAUBER, MACKIN1966) caused mortalities as high as 95% on transplanted oysters throughout Delaware Bay (SINDERMAN and ROSENFIELD 1967). Coupled with high levels of pollution, overfishing and inadequate management MSX virtually ended all commercial forms of oystering in Delaware. The oyster producing tributaries remained closed to oystering until 1965 when they were opened for tonging of seed. Because these rivers had been closed for such a long period the condition of the populations was unknown.Inl968 *Field Station, College of Marine Studies, University of Delaware, Lewes, Delaware Received February 28, 1972

and 1969 a systematic survey of Delaware's oyster industry was conducted (MAURER et a1 1971). The survey included Delaware Bay and the rivers. The major goals of the study were to determine the distribution, condition, and abundance of oysters, to relate the differences between oyster populations to environmental factors and finally to recommend various procedures for the rehabilitation of the oyster industry. During the course of the survey a relationship between oyster beds and river morphology was observed. This paper presents data on the effect of tidal stream formation on larval setting spat development, and maintenance of a healthy adult population. Results indicate that there is a strong association between oyster beds and the meanders of rivers.

The four major oyster producing rivers in the state of Delaware are: the Leipsic, Murderkill, Mispillion, and Broadkill (Figure 1). In the majority of rivers the oyster producing sections extend 35004000 yards (3150-3600 m) from the river mouths. These sections were staked off in 100 yard (90 m) intervals. An oyster dredge with an approximate one meter mouth was towed over the right, middle, and left portions of each 100 yard (90 m) section. All live oysters (larger than 2 cm in height) were counted and measured to the nearest tenth of a centimeter. The total number of oysters per haul was used to plot the density of the oyster beds, and eventually to estimate the total number of bushels for each river. MAURERet a1 (1971) contains further details on methods.

Densities of oysters for the Leipsic, Murderkill, Mispillion, and Broadkill Rivers are plotted in Figures 2-5 respectively. The majority of oysters occur on the large meanders with the optimum areas on the down stream outer edges. Numbers of oysters (under 6 cm in height) that represent set from the preceding two years also occur on the meanders and are summarized in Table I. In contrast some areas (Station 1-4) in the Murderkill River contain dense populations on the inside of the meanders (Figure 3). The first meander of the Mispillion River (Stations 10-15) also appears atypical compared to the distribution pattern of the other rivers.

Fig. 1. Map of Delaware Bay showing location of major oyster producing tidal rivers.

Tidal regions are characterized by areas of scour and deposition

(BREHMER 1965). Productive oyster beds are commonly located in areas free from soil deposition. Currents are effective in the preparation of cultch because they prevent the deposit of sediment and slime which renders shell unsuitable as a setting surface (MOORE 1897). Shell placed in areas of relatively high current velocity will tend to collect more spat than low velocity areas because a greater

(

LElPSlC RIVER ,.cr.

I v3

OYSTER DENSITY

Fig. 2. Map of Leipsic River showing oyster densities. Arrow indicates direction of ebb tide.

Fig. 3. Map of Murderkill River showing oyster densities. Arrow indicates direction of ebb tide.

372

TABLE I Number of oysters under 6 cm, representing recent set at each station. Station Number

Leipsic

Murderkill

Mispillion

Broadkill

volume of water and hence a higher number of larvae will contact the shell. For example, PERKINS (1952) found oyster larvae in heavy concentrations where the current was greatest and the salinity showed no stratification. The development of oyster bars can be associated with the formation of tidal streams. The transport of sand in opposite directions during the ebb and flood causes meanders in mature stages of stream development (SCHOU1967). Jakobsen (1962) provides the background for the formation of ebb and flood channels in these streams. According to him an ebb channel is normally open to the ebb current and contains a bar at its mouth while a flood channel is generally open to the flood current and has a bar at the upper end. Stream morphology of tidal areas is mainly caused by tidal currents and most sections are dependent on either the ebb current or the flood

1

MISPILLION RIVER

OYSTER DENSITY

4

Fig. 4. Map of Mispillion River showing oyster densities. Arrow indicates direction of ebb tide.

BROADKJLL RIVER

OYSTER DENSITY number/ 100 yds.

182.8m.

Fig. 5. Map of Broadkill River showing oyster densities. Arrow indicates direction of ebb tide.

374

current. The presence of bars at the mouth of the St. Jones, Murderkill, and Broadkill Rivers together with stronger ebb tides than flood tides indicate that Delaware's tidal channels are generally formed by the ebb current. KRAFT(1971) commented that the area ofthe St. Jones and Murderkill Rivers was poorly drained and contained depressions of interior drainage similar to those found elsewhere along the Atlantic Coast. KRAFTstated that the origin of these has not been satisfactorily explained. The meanders formed in this process develop an undercut bank on the outside of the bend termed the undercut slope and deposits of alluvium on the inside bend termed the slip-off slope. Although these terms are normally applied to river development in classical geomorphologic situations (STRAHLER l969), rather than tidal streams, they are useful descriptive terms and are retained here. Some general stages of oyster bar development as related to the formation of tidal streams are depicted in Figures 6 through 8. I n stage I (Figure 6) erosion occurs at such a rapid rate that oyster bars cannot maintain themselves. However, existing oysters would be randomly distributed across the bottom. With the advent of meanders in stage I1 (Figure 7) oysters have a good opportunity for setting. Data (Table I) presented herein show that there is a predominance of seed oysters on the outside of large meanders. This supports the view that these areas make ideal setting sites because of increased water velocity and its scouring affect (ANDREWS, personal communication). Figure 7 depicts areas of scour (A, undercut slope) which enhance good setting and high survival. Point bar deposits (Figure 7, B) on the slip off slope cause deposition or siltation and thus become a poor setting area. Adults also may be suffocated. Evidence (CRISP1967, HIDU 1969, BAYNE1970, KECKet al. 1971, VEITCH& HIDU1971) has accumulated that larvae are stimulated to

Fig. 6. Diagrammatic cross-section of a tidal stream at stage following primary vertical down cutting. See explanation in text.

Fig. 7. Diagrammatic cross-section of a tidal stream at stage of meander formation. See explanation in text.

set due to release of water soluble pheromones by other oysters. Research in progress hy -this.litbo-ratoryindkates that pheromones may serve as active attractants for setting rather than as setting stimuli. Regardless of the difference in views, oyster larvae tend to cluster where adults are present assuring continued survival of the population (CRANFIELD 1968). In stage I11 (Figure 8) meander formation has become very advanced resulting in point bar deposits on the slip-off slope which commonly are intertidal in their upper levels. These intertidal areas (Figure 8, C) provide excellent setting sites in terms of survival of high numbers of juveniles. In Figure 8 areas A and B have good and poor setting and survival for reasons discussed above. Intertidal areas at the mouths of the Murderkill and St. Jones rivers commonly yield high numbers of spat. The latter river, which is sadly depleted and was not included in this account for that reason, still contains intertidal areas with relatively high counts of spat. We attribute this distribution in great part to the absence of predators and fewer fouling organisms which compete for food and surface area with young oysters. Based on our own research and that of HOWARD MARSHALL (personal communication) we found that artificial airing of fouling organisms on oysters drama-

Fig. 8. Diagrammatic cross-section of a tidal stream at stage of advanced meander formation. See explanation in text.

tically reduced the incidence of fouling. Artificial airing essentially simulates the principle of intertidal areas which involve diurnal exposure to air. I n the case of artificial airing, however, exposure time of fouling organisms is systematically controlled. Because of reduction of predation pressure the establishment of intertidal oyster beds has been favored in areas where temperature permits. The distribution of intertidal beds is certainly better developed in southersareas (eg.,+Sou.th-Carolina, Alabama, Lou+ana) with milder winters than in areas north of Delaware. In addition a similarity between oyster bars in Delaware's coastal rivers and Louisiana's long and narrow channels (bayous) was brought to our attention (GUNTER-personalcommunication). However, the situation in Delaware has been disturbed by human activities. Exceptions to the development of oyster bars by the process outlined above are cited in HEDGPETH (1957). He notes that oyster bars may gradually acrete from the shoreline or that they may construct bars transverse to the river or even parallel to the channel. Locally we found the Mispillion River to vary from the pattern of other rivers. This was particularly evident between stations 10-15. One reason which may have caused this deviation is fishing pressure. Earlier it was mentioned that the rivers had been closed to fishing approximately 15 years. This assured us that the survey should be relatively unaffected by fishing pressure. However, the Mispillion has been irregularly harvested on a larger scale than the other rivers (Delaware Commission of Shellfisheries Annual Reports). I t was noticed that dense populations of oysters appeared displaced in the down stream portion of the meanders. DEWITT(1968)reports that these shallow, tidal rivers have a short flushingrate of 7-14 days. Delaware Bay, with slower flushing rates immediately adjacent these rivers act as a larval trap as suggested by BEAVEN (1959). I t may be reasoned that a good set in Delaware's oyster rivers results from the exchange with the bay of larvae laden waters on flood tides. The fact that densities of adult and seed oysters are highest on the meander nearest to the bay may probably be explained for the same reason. In summation the distribution of oyster bars in Delaware's tidal rivers is highly dependent on the interaction between geological and biological features. These features involve meander formation and the ability of oyster larvae to select a substrate with a high potential for survival.

We would like to acknowledge Dm. J. ANDREWS, G. GUNTER, and J. C. KRAET who reviewed the manuscript and offered comments. We would also like to thank for preparing the illustrations. This research was supported in MRS.ANNTAYLOR part by P.L. 88-309, the National Science Foundation Sea Grant Program, the Delaware Department of Natural Resources and Environmental Control.

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